Bleaching a tooth, or providing it its original natural, specific to the patient, has been an action commonly carried out in dentist's surgeries for more than 100 years and is very significant for the psychological balance of the patients.
A very good explanation for this aesthetic action was given, and constitutes a reference in this matter: the special addendum to JADA (Journal American Dental Assoc.) of April, 1997, No. 128 (addendum) pp. S1–S64 entitled “non-restorative treatment of discolored teeth, reports for an international symposium” and summarizing the congress of Chapel Hill, North Carolina of Sep. 25 and 26, 1996. An update can be found in CRA (Clinical Research Associates newsletter) Vol. 24, Apr. 4, 2000 issue or, more recently, in “Incidence of tooth sensitivity after home whitening treatment” by Jorgensen and coll. JADA, August, 2002, Vol. 133 pp. 1076–1082.
From this work can be seen that there are nowadays three important methods of bleaching used in the dentist's and medical surgeries:                a mechanical method consisting in associating to the plaque control of teeth by mechanical means (manual and ultrasonic) abrasive polishing pastes.        a chemical method, in general following the previous one, and consisting in applying to the tooth a product capable of removing the surface deposits as those due to tea or coffee. These products are very low-concentration carboxide- or peroxide-based products, and can be used by the patient himself at home.        a more invasive chemical method having higher concentrations of peroxide-based product, often requiring the dental surgeon's intervention, taking into consideration the risks incurred by the patient if it does not follow its therapeutics according to the rules of the dental and medical field, and allowing to reduce the coloration of the teeth inside the dental body itself. Recently this method was modified and reduced in action in order to be usable by the patient himself at home (home kit) under cover of a periodic medical control.        
Unfortunately and very quickly, both the practitioners and their patients realized that:                the application time was very long and required the immobilization of the patient during more than 5 minutes per tooth, or 20 minutes per half arch.        the cost of the intervention was therefore painful and prohibitive!        
For this reason products were developed reacting more quickly while being activated by the light or heat. This method allows to reduce by four the time spent for the medical bleaching action.
Based on these promising results a number of products known as photosensitive were put on the market and used abundantly and successfully, associating perborate and hydrogen peroxide or peroxide carbamide activated by camphoroquinone, itself photosensitive between 400 and 500 nm. These products result from the techniques developed and introduced originally by CORCORAN and ZILLICH (1974) and by RENNEBOOG (1989). These studies highlight the role of heat and radiation brought by the halogen lamps in the activation of the bleaching products.
Thus, nowadays there exist on the market many bleaching products that are directly usable by the patient at home or that can be applied at higher concentration by the dentists. These products act directly or after activation by light or heat. They use in great majority as basic formula hydrogen peroxide at approximately 35% as described for first time by HALON in 1884.
To activate even more the reaction and to further divide by two the already considerably reduced time, it was asked to develop even more powerful lamps and it is for this purpose that the xenon-arc plasma lamp “Apollo 95E”, patents FR 2,773,986 and FR 2,782,000, was invented and developed, which included a “bleaching” function and which supposed an action of about 30 seconds on the product placed in contact with the tooth. This product is often in the form of a gel maintained in a transparent gutter. A typical example has been sold for a long time under the name of “Apollo secrete whitening kit” (DMDS Corp. Los Angeles, USA).
Admittedly the results obtained were spectacular and many manufacturers followed this technology. However this process, even if it reduced the time considerably, had many limits.
It was indeed shown that:                with some products the action of the lamp, therefore its effectiveness, was not only due to the photonic, but also to the thermal emissions and it is under these two effects that these products were activated.        since the cost of these lamps is very high, the treatment remained relatively expensive,        since these methods require high peroxide concentrations, they impeded their use by the patient at home,        the thermal rise observed in the tooth was disproportionate compared to the activation of the product and could even be dangerous when the action was too long,        it was impossible to properly control the thermal value at the very level of the tooth, to the risk of causing significant disorders in the health of dental pulp itself. Moreover, the movement even of the hand could result into changes in localization of the point of luminous or thermal impact.        by replacing the light emitted by the lamps with high thermal emission, such as the xenon-arc or halogen lamps, by lamps known as cold lamps, such as those described in FR 2,805,148 and FR 2,318,892, the heating effect was removed, which allowed the operator to increase without any risk his time of action to activate the photosensitive products, but in parallel obliged him again to a long exposure because of the elimination of the source of heat.        if a dispersed light is used over the whole arcade (for example in the form of luminous gutter), the time is again reduced, but not more than by using the high powers.        the doses implemented for treatment in the dentist's surgery (30% peroxide) as well as at the patient's home (8–10% peroxide) are extremely high compared to the aim searched for and explains the side effects observed (dental pains with the cold).        there are many repetitions because the molecules responsible for coloration are modified, even cut, but never actually removed from the site. The fact that they remain allows the recombination of the initial chemical bonds explaining the need for a re-treatment at increasingly closer intervals.        finally, the aggressive action of peroxide in contact with the gum always obliges the practitioner as well as the patient to many precautions to avoid the bums during and after the treatment.        
Moreover, at no time has been solved a fundamental problem which justified the putting under monitoring of the bleaching products and in particular the peroxide by all the safety committees in the field of health of the EC and the FDA, irrespective of the degree of degradation of the tooth itself under the effect of these products, whether they are massively applied in high doses, or slowly at home by the patient himself. Do the post-operational pains result from a degradation of the tooth such as many authors affirmed? How to correct some significant decalcifications observed among patients after erroneous applied treatments?
Finally, though it is possible through known techniques to allow a faster action of the bleaching products, there is no method allowing to control, at low cost, the activation of the chemical bleaching components and to correct the degradation of the tooth following their effects.
Even more serious is what will happen with the molecules responsible for coloration. Indeed, after a treatment primarily based on the division of large colored molecules into smaller ones, the residue of this reaction remains, with the active agent, inside the dental body, which cunningly continues its action and significantly limits the penetration of fluorine or calcium.
In addition to the consequences of this steric space occupation, it can be seen well that the arrival on the site of intra-dental action of the bleaching molecule occurs by pure passive permeability, undoubtedly explaining its weak penetration, the time necessary to allow its action, the absence of a possible control of the reaction by the operator, but also a total lack of knowledge of its actual concentration in the area of action, i.e. on the molecules responsible for coloration in dental tissue.
Finally, the absence of objective and inexpensive reference to the beginning and the follow-up of the evolution of bleaching of the tooth makes its estimate perfectly subjective.